1. Field of Technology
Precision saws primarily for orthopedic surgery and the blade assemblies used in such saws.
2. State of the Art
Precision cutting of bone is a fundamental requirement for orthopedic surgery. Reciprocating or rotating blade saws often are the tools of choice in these applications. Many reciprocating tools use single, narrow blades with cutting teeth along curved cutting edges. The blades reciprocate over a small arc. One drawback: the single reciprocating blade creates undesirable action/reaction forces. As the blade reciprocates in one direction, forces from the bone to the blade and through the tool to the surgeon's hand push in one direction on the hand. When the blade reverses direction, forces on the surgeon's hand also reverse direction and vibrate his or her hand. The alternating directional forces or vibrations makes controlling the cutting edge's position and movement more difficult.
Ideally, cutting should be controlled, two-directional chipping in which bone removal approximates the width of the cutting blade. Without proper control, however, the blade can move out of the desired two-dimensional cutting plane. The cutting tip, which is where the cutting edge of the blade contacts the bone, essentially vibrates in three dimensions. Bone cutting becomes a more violent chipping into and out of the ideal cutting plane.
Even circular saw blades that rotate in one direction create problems because reaction forces are difficult to control. As the surgeon contacts bone or other tissue or changes the applied forces, the reaction forces on the blade change. The variable reaction forces cause loss of cutting control.
The geometries of conventional cutting blades limit cutting to a small front arc of the blade. As the arc through which cutting takes place increases, the effect of the action/reaction forces also increases. However, using a small arc limits cutting to areas that the cutting surface of the blade conveniently reaches. Other bones or tissue adjacent the bone being cut may block the cutting surface from reaching the cutting zone.
Some problems with using blades with a small arc stems from the problem of the forces on a single blade. Overcoming the single-blade problem could make small-arc blades more viable.
Instead of using a single reciprocating blade, these saws use two, counter-reciprocating blades. The saws may use three blades, with the top and bottom ones reciprocating together and the center one counter-reciprocating with the top and bottom ones.
Two-bladed electric carving knives for carving meat, poultry and other food are well known. They provide substantial control and fast cutting. Typical electric carving knives have two flat, usually serrated, blades that reciprocate along the plane of the cutting surface. As one blade moves outward, the other blade moves inward. Most have some mechanism for holding the blades together while they reciprocate.
Cripe, U.S. Pat. No. 5,846,244 (1998), discloses a counter-reciprocating surgical saw in which the saw teeth move in an arc.
For quality cutting of bone along a single plane, the counter-reciprocating blades should stay together, but they naturally vibrate apart especially while cutting bone. When the blades move apart, they fail to make a single cut in a controlled plane. In addition, cut bone enters the space between the blades, which keeps the blades apart and interferes with cutting.
Familiarly shaped saw housings such as the common pistol shape can accommodate mechanisms for reciprocating the blades. Many reciprocating mechanisms vibrate and make noise. Vibrations tend to move the blade assembly during cutting, which makes controlled cutting more difficult. Vibrations and their noise also are tiring to surgeons and can cause injury to surgeons' hands over time. In addition, running quietly during surgery can be important because loud noises are draining to the operating room staff and to patients if they are awake.